The present invention relates to a method for producing a blank for a gear rack for rack-and-pinion steering of motor vehicles, and a stamping apparatus for carrying out the method.
German Published Patent Application No. 197 26 697. In this Patent, a method for producing a gear rack is. The blank for producing this gear rack has a shape differing from a circular cylinder at least in the region of its tooth system. Hitherto, such a blank could be produced, for example, by a machining process, by extruding, or by another forming process. During such a forming production process, considerable strain-hardening could occur in the blank in various regions. However, subsequent non-cutting forming processes are made considerably more difficult by such strain-hardening. It was possible to remedy this by introducing additional annealing processes between the individual forming processes. However, such heat treatment processes require an interruption in the cold forming from the starting material via the blank to the finished gear rack.
Further processing in a non-cutting method is described, for example, in German Published Patent Application No. 32 02 254. In this method, a gear rack is produced from a cylindrical starting material by orbital forging between a top die and a bottom die. In this method, too, strain-hardening may occur in the region of the tooth system of the gear rack, and this strain hardening makes the production of the gear rack more difficult.
Japanese Published Patent Application No. 57-206546 describes a stamping apparatus for forming a gear rack from a hollow-bar blank filled with an inherent substance during the shaping. The gear rack is provided with at least a partial tooth system. The stamping apparatus includes a plurality of form tools that are movable relative to one another and provide a forming effect on the hollow-bar blank due to the forces and reaction forces occurring in the process.
U.S. Pat. No. 4,715,210 describes a die for forming a Y-shaped gear-rack part by forging. The die includes a group of four forming elements that are arranged symmetrically about the longitudinal plane that corresponds to the plane of symmetry of the gear-rack part to be formed. At least three of these forming elements are movable relative to one another.
In the foregoing stamping devices, the form tools required for the shaping operation act on the gear-rack blank from different directions. Thus, forces from different directions must be compensated for by the apparatus. Housing parts of the apparatus must therefore be larger. In addition, the driving devices for the forming elements must also be relatively complicated and costly, which further requires a larger size of the apparatus.
It is an object of the present invention to produce a blank for a gear rack in such a way that, in particular in the region of the gear rack tooth system to be produced subsequently, only slight strain-hardening occurs. This region is therefore of great importance, since considerable flow movements of the material occur during the subsequent forming process, for example, an orbital process. Overall production costs may be sharply reduced by the method.
In the method according to the present invention for producing a blank for a gear rack, using a stamping apparatus with at least three form tools which are movable relative to one another, in a first step, a starting material with an essentially circular cross-section is positioned between the form tools. In a second step, at least one force in at least one direction is then brought to bear on at least one of the form tools, by which the form tools move relatively to one another. Thus, the starting material is reformed into a blank for the gear rack which, at least in the axial direction of the gear rack tooth system, takes on a shape differing from that of a circular cylinder. Through the introduction of a force to at least one of the three form tools movable relative to one another, without major strain hardening, the starting material may be formed into those regions of the blank which experience greater deformations in the further course of producing the gear rack. An additional advantage of this method is that in a simple way a blank form can be achieved, with which the formation of burrs can be avoided during a subsequent reforming process, in particular with an orbital process.
The stamping apparatus according to the present invention may be used for performing this method. This apparatus includes at least three form tools which are movable relative to one another, of which one is fixed, while the other two are movable relative to each other and to the fixed form tool. Two of the form tools include effective surfaces, inclined to each other, which lie symmetrically to a plane of symmetry of the gear rack. This plane of symmetry is perpendicular to the tooth system of the gear rack, and contains the longitudinal axis of the gear rack. The third form tool includes an effective surface which extends perpendicular to the aforementioned plane of symmetry of the gear rack.
Using such a stamping apparatus, and with the use of the method according to the present invention, a blank for a gear rack may be produced without having to overcome major frictional forces during reforming from the starting material, and without the appearance of significant strain-hardening in critical regions.
Thus, during the second method step, a convex or concave longitudinal profile may be worked into the starting material, at least in the axial region of the gear rack tooth system at the circumference of the blank outside of the tooth system, the longitudinal profile, during further method steps, being used to absorb the orbital moments. During the second method step, in the tooth system region of the gear rack, the blank is provided with two surfaces positioned essentially symmetrical to each other, which can be flat or vaulted. The longitudinal profile may have various cross-sections, such as, for example, a circular segment, a rectangle, a triangle or a combination thereof.
In the stamping apparatus it is advantageous if only one of the three form tools is fixed. In this regard, the fixed form tool may be either one of the two form tools that are symmetrically arranged to the plane of symmetry, or it may be the form tool, the effective plane of which is arranged perpendicular to the plane of symmetry of the gear rack. If the two movable form tools are those that are symmetrically arranged with respect to the plane of symmetry of the gear rack, then these two form tools are each acted upon with the same force and the same speed in a direction perpendicular to the plane of symmetry of the gear rack. On the other hand, if the form tool, the effective plane of which extends perpendicular to the plane of symmetry, is one of the two movable form tools, this form tool is coupled with the other form tool so that it moves at half the speed of the other movable form tool.
In both cases this achieves that the plane of symmetry remains unchanged in its position opposite the form tool having its effective plane extending perpendicular to the plane of symmetry. Since the counterpart to the concave or convex longitudinal profile of the blank, respectively, is contained in this form tool, the longitudinal profile always remains in the plane of symmetry of the gear rack.